In various types of indoor environments, such as office premises, receptions and reception halls, production premises, sports halls and indoor swimming pools, playgrounds and classrooms, it is desirable and also statutory according to regulations, to provide good acoustic conditions to the environment. Acoustic conditions can be best described by the reverberation, and to control this, sound absorbing elements are used, such as sound absorbing panels attached to walls, ceilings, and other surfaces.
Sound absorbing panels as surfaces for attachment to indoor walls and ceilings come in types that use various physical effects for the absorption of sound. Firstly, there are so-called fibre absorbents. These comprise porous panels of mineral fibres (rock and glass wool), which dampen sound as the sound waves penetrate into the panel, and the energy of the sound waves is reduced by viscous losses in the pores and absorbed by the fibres as heat.
Furthermore, there exist absorbents, which are based on the Helmholz resonator principle. Such panels generally include slits or apertures, and require fibre fabric or porous fibre materials behind the panel to obtain satisfactory absorption. Normally, a fibre fabric is used, but this is often combined with thicker fibre mats to obtain better absorption. In the latter case, the fibre fabric is often integrated as a surface layer on the fibre mat.
Another type of absorbent is membrane absorbent. The most common type is thin panels of metal, such as steel or aluminium, or of plastic, which is mounted at a certain distance from a wall or ceiling. A special type is disclosed in patent publication U.S. Pat. No. 5,719,359. Here, the sound is absorbed as the sound energy creates movement in a membrane, in the form of thin strips. The general problem with membrane absorbents is that the resistive component, which makes them function as an absorbent, is small, and moreover, is almost impossible to estimate. This is partially solved by arranging the strips against each other, yielding friction as they move as a result of the sound.
Patent publication U.S. Pat. No. 4,821,841 discloses a panel element for sound absorption, with a panel with slits arranged over a rear plate. The slits are approximately 1.6 to 19 mm wide, and the panel element is adapted to have fibre material arranged in the space between the panel with slits and the rear plate, to obtain the desired absorption.
There are various weaknesses with such fibre-based sound absorbing panels. An important one is that they produce fibres to the environment in the event of damage or wearing. Such fibres are often made of melted glass or rock, and give the sensation of dry air and irritation of the respiratory passages of persons in such environments. Furthermore, these fibres limit the appearance of such plates. It is difficult to keep them clean as they require minimum use of moisture when cleaning, and problems related to mould and decay may arise, especially in rooms exposed to moisture, such as kitchens, indoor swimming pools and the like.
Another type of panel avoids these drawbacks by using friction by viscous airflow to dampen the sound waves. Such known panels comprise microperforations, i.e. holes through the panel of diameters less than 0.5 mm. These panels are not dependent on fibre materials. The panel is arranged with a distance from a rear surface, in such way that an air space is formed between the microperforated panel and the rear surface. As the sound waves hit the panel, the air in the perforations is forced back and forth due to the pressure differences resulting from the sound waves. This movement results in viscous friction, by which the energy in the sound waves is converted to heat, whereby the sound waves are dampened.
Such a sound absorbing panel element is disclosed in patent publication WO 03001501. This panel element is intended for sound isolation of car engines and the like, but can also be used as sound absorbing elements in buildings. The panel element consists of a panel with microperforations, arranged at a distance to a rear surface, with the perforated panel facing the sound source. This panel element avoids the disadvantages of fibre based sound absorbents, as described above.
Microperforated panels and foils are in many cases produced by rolling a tool with many small spikes over the surface or the foil. Other methods, such as laser cutting and plastic moulding are used for thicker panels and for panels of other materials.
A need in the market for new absorbents that allow for architects' desire for a clean and smooth surface is identified. With its low perforation level and special design, the present invention provides a solution to the market, which complies with this need. Products based on the invention can be adapted to the individual customer's needs concerning surface finish, shape, and choice of material.